The email comes from a trusted source -- the CEO, a regular vendor, the company attorney or accountant. It's part of an ongoing conversation, the format and language is identical to previous emails of the same type.

There might even be phone calls. It's no surprise that in this situation an employee would send a wire transfer to a new payee or a sensitive business document to someone who turns out to be a fraudster.

According to ZapFraud, these kinds of spear phishing attacks, known as business email compromise (BEC), now account for 4 percent of the total volume of scams, up from from less than 1 percent in 2011.

BEC fraud was the fastest-growing type of spear phishing attack last year, reported PhishLabs in a report released late last month.

This type of fraud cost global business more than $1.2 billion over the past two years, the FBI reported late last summer, and the number of victims increased by 270 percent during the first eight months of 2015. The average loss per scam was $130,000.

According to the FBI, BEC fraud typically includes urgency or secrecy, uses a look-alike email address, takes place just before a holiday or weekend or while the CEO is away from the office, and typically results in a large sum of money transferred to China.

Some of these aspects are evolving, however. Fraudsters increasingly spoof the actual email address of the CEO or trusted business partner, and ask for the money to be sent to a domestic bank account, according to PhishLabs.

This scam is both very profitable and difficult to defend against using traditional defenses, said Markus Jakobsson, ZapFraud's founder and CTO.

"Spam filters look for large volumes of similar or identical messages," he said. Spear phishing emails are individually targeted, so they slip past all the spam filters.

And the return email addresses are also individually created for each scam, so they don't show up on blacklists, he added.

Other usual signs of spam, such as outlandish stories about lottery winnings or Nigerian princesses, or keywords like Rolex or Viagra, also don't appear anywhere.

"It looks like it comes from someone you trust, and it's about everyday business," said Jakobsson.

Improving business processes and educating employees can help, he said.

"For example, one policy could be to never wire money unless you see the face of the person requesting it, and they're in your office," he said.

This could be particularly effective for high-value transactions to new payees.

However, it might not be as effective for scams where the fraudsters are looking to get copies of sensitive documents instead.

"If you have to show up in the office for every document, that would take us back many many years in terms of efficiency," he said.

For example, messaging service Snapchat and Segate, a disk drive manufacturer, recently fell victim to BEC emails that tricked them out of tax documents that contained employees' addresses and Social Security numbers.

There are technical solutions as well, Jakobsson said, including those offered by his company.

For example, a filter could check whether an email address is close to -- but not identical to -- that of a trusted contact.

"Or it might have a different reply-to address from the return address -- which is easy for attackers to spoof for companies that don't use DMARC," he added.

DMARC is a 4-year-old project that helps companies authenticate email addresses. According to a report released last month by email security vendor Return Path, only 29 percent of global brands use DMARC.

"If it's an actual account takeover, the best approach is to confirm on a different channel, such as an SMS or known alternate email address," he said.